1. Field of the Invention
The invention relates generally to image data processing. The invention relates more specifically to a low cost method for providing shadowing, highlighting and other image processing functions in a system which processes images in the form of digital signals.
2. Description of the Related Art
In recent years, the presentation and pre-presentation processing of visual imagery has shifted from what was primarily an analog format to an essentially digital format. Unique problems come to play in the digital processing of image data. The problems include providing adequate storage capacity for digital image data and maintaining acceptable data throughput rates. In addition, there is the problem of creating a sense of realism in digitally generated imagery, particularly in animated imagery.
The visual realism of imagery generated by digital video game systems, simulators and the like can be enhanced by providing special effects such as shadowing, highlighting and so forth. For example, when the image of an airplane is to be displayed flying over a flat terrain on a sunny day, the realism of the overall scene is enhanced by generating a shadow image of the airplane within the image of the terrain. The effect appears more realistic when the terrain region onto which the shadow is projected becomes dimmed rather than completely blackened. The observer continues to see part of the texture of the terrain even though it is covered by the airplane's shadow. The effect is referred to as "shadowing."
Highlighting is another example of realism-imparting effects. Suppose an explosive device is displayed detonating near the airplane. Visual realism is enhanced by momentarily increasing the brightness (highlighting) of the airplane's image to create the impression that light from the explosion is reflecting off the airplane's fuselage. The effect appears more realistic when certain brightness and/or colorization relationships between different parts of the airplane (e.g., cockpit, fuselage, wings) are maintained.
Home game systems, such as the Sega "Genesis" have a two-source image merging system for creating shadowing and highlighting effects on-the-fly (in real time). When the shadow effect is desired, a first source "tile" or "sprite" (rectangular block of bit-mapped data representing the airplane shadow region) is overlaid with a second source tile representing the underlying terrain. For every pixel position where the first (shadow) sprite intensity is non-zero, a digital signal representing the corresponding terrain intensity in the second tile is cut in half to thereby produce a "dimming" effect. The dimmed version of the terrain tile is then output as part of the video image. (The dimmed image data is not stored in memory however.)
To produce the highlighting effect, the Sega "Genesis.TM." system divides the shading value of all pixels within the airplane's sprite by two and then adds half the maximum shading value to each such pixel. This preserves the relative shading relation between parts of the airplane while making each brighter. The augmented version of the airplane tile is then output as part of the video signal, but not saved. It is not possible to both shade and highlight a tile at the same time in the Sega "Genesis.TM." system.
The above-described shadowing and highlighting techniques are of limited use. Optically-complex animated scenes require much more. Consider for example a scene in a Knights of the Realm kind of game. The hero enters the arch chamber of a church. Stained glass windows of different elevations, colorations, transparencies, shapes and angles surround the chamber. A villain is to be seen through the stained glass windows, approaching from the outside of the chamber at an angle relative to the stained glass windows. The scene is to be projected through, or displayed on, a two-dimensional window (hereafter, the observation plane). For added realism, the position of the observation plane (the window through which the game player views the scene) is to rotate slowly about the hero, thus giving a three-dimensional quality to the displayed two-dimensional scene.
Realistic rendering of such a scene has to take into consideration the transformation of outside light as the light passes at various angles through the stained glass windows to the observation plane. It also has to take into account the reflection of internal lighting off the stained glass windows toward the observer's plane. Moreover, if the villain throws a rock through one of the stained glass windows, the visual effects of the hole have to replace those of the removed window material. If the villain flings mud onto a window, the transparency and coloration of the affected window regions have to change accordingly.
Previously available home-game systems (e.g. Nintendo Entertainment System.TM., Sega Genesis.TM.) were not capable of handling such optically-complex animated scenes in real time. Some commercial imaging systems such as the Silicon Graphics "Iris.TM." system do provide mechanisms for handling optical complexities in real time, but this is made possible only through the use of high speed computers, large memories and special custom circuitry. These commercial systems are therefore available only at very high cost.
Heretofore, a low cost system for providing realistic renditions of complex animated scenes has not been available.